📅 OpEx Dynamics

OpEx — monthly options expiration — falls on the third Friday of every month. A large fraction of open interest rolls off that day, which mechanically reshapes the gamma board. The two effects worth knowing are pinning into OpEx and the post-OpEx drift.

Pinning into OpEx#

When a strike has dominant open interest and dealers are long gamma there, their hedging is mean-reverting. As price drifts above the strike they sell; as it drifts below they buy. The result is often a tight range around the strike into Friday's close.

Look at the GEX heatmap on Thursday afternoon of OpEx week. A single dominant strike near spot with heavy call-and-put gamma is the pinning signature.

The post-OpEx drift#

After Friday's close, the bulk of expiring open interest is gone. The board re-weights toward later expirations, and because dealers often held net long gamma in the expiring tenor, aggregate gamma usually drops. Less mean-reverting hedging = bigger realized moves the following week — this is the widely observed post-OpEx weakness effect.

How to read it on GammaBaba#

Open the GEX Multi-Panel during OpEx week to compare the expiring tenor against the next two.
Use the GEX Flow panel over Friday → Monday to quantify how much aggregate gamma rolled off.
On Monday, check whether the new King Strike is meaningfully far from spot — that extension often sets the week's tone.

OpEx effects are stronger on single stocks than indices, because indices now have heavy weekly and 0DTE flow that dilutes the monthly cycle.

Looking for exact dates? See the 2026 Options Expiration Calendar — every monthly OPEX, VIX, SPX/XSP weekly, EoM/EoQ and exchange holiday in one color-coded reference.

🔑 Key Takeaways

OpEx = 3rd Friday — major open-interest roll-off.
Pinning: dominant long-gamma strike + spot near it = tight Friday range.
Post-OpEx drift: gamma drops, realized vol typically expands.
Single-stock OpEx effects > index OpEx effects (due to 0DTE/weeklies).

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